Two-dimensional Spread Spectrum System Code Key Techniques Of Differential Detection

Time-frequency domain two-dimensional spread spectrum (2D-SS) is a novelsolution for spread spectrum communication. The operation of spreading spectrum isperformed not only in time domain but also in frequency domain in2D-SS. Relative tothe traditional directed sequence-code division multiplexing access (DS-CDMA) systemand the multi carrier-code division multiplexing access (MC-CDMA) system, the higherprocessing gain and greater multiple access capability can be obtained in2D-SS system.Besides, its anti-fading and low probability of intercept (LPI) characteristics makes it ofa very important research value. As a non-coherent detection scheme, differentialdetection also deserves to be studied for its no needs for channel estimation andequalization at the receiver, which will simplify system design and reduce costs.Differential detection can be used for demodulation in fading channel, for theaccuracy of channel estimation can’t be guaranteed. Thus, we combine2D-SS systemwith differential detection technology in this thesis and research on some keytechnologies of chip-level differential detection in2D-SS system.First, three different chip differential coding patterns and detection methods havebeen designed. The analytical expressions of the bit error rate (BER) of the threeschemes have been given respectively. Meanwhile, channel parameter changes effectson the BER performance of the system have been discussed by means of MATLABsimulation. Including:(1) A time domain chip-level differential detection scheme in2D-SS system hasbeen proposed. Theoretical analysis and simulation results reveal that the schemeis able to counteract the frequency-selective fading of the channel and the BERperformance becomes better with the decreasing of the channel coherencebandwidth. This scheme exhibits a5dB better BER performance in typical urban(TU) channel than four path fading channel with BER reaching at10-4.(2) A frequency domain chip-level differential detection scheme in2D-SS systemhas been proposed. Theoretical analysis and simulation results show that thescheme is able to counteract the time-selective fading of the channel and the BERperformance becomes better with the decreasing of the channel coherence time.This scheme exhibits a2.5dB better BER performance in normalized fadingbandwidth f_dT=2fading channel than f_dT=1fading channel with BERreaching at10-5.(3) A time and frequency domain chip-level differential detection scheme in2D-SSsystem has been proposed for frequency selective fast fading channel.Theoretical analysis and simulation results display that the scheme is able tocounteract both the time-selective fading and frequency-selective fading at thesame time and the BER performance becomes better with the decreasing of the channel coherence time or coherence frequency. This scheme exhibits a4dBbetter BER performance in typical urban (TU) channel than four path fadingchannel with BER reaching at10-4and a5dB better BER performance innormalized fading bandwidth f_dT=1fading channel than f_dT=0.1fadingchannel with BER reaching at10-3.Second, a multiple-symbol chip-level differential detection method is proposed.Under the premise of not changing the system spread spectrum gain and bandwidth, theBER performance in fast fading channel can be improved by adjusting the size of thetime domain spreading factor and the frequency domain spreading factor to increase thenumber of sub-OFDM symbols and reduce the number of subcarriers, which willdecrease the correlation of inter carrier interference (ICI). Theoretical analysis andsimulation results show a2dB better BER performance in system using this method.Finally, an increasing of the detection region size method is proposed. Under thepremise of not changing the system spread spectrum gain and bandwidth, the BERperformance can be improved by selecting a larger size of_detection region within thecoherent region size. Theoretical analysis and simulation results reveal a1dB betterBER performance in system using this method.The research in this thesis explores some key technologies of chip-level differentialdetection in2D-SS system, such as design of_differential patterns, design of_detectionmethods, selection of spreading factor and selection of_detection region. The researchresults are not only of important theoretical value but also provide a reference forengineering applications.